The invention relates essentially to the POD method, i.e. a technique of plasma deposition by spraying and melting a silica powder on a primary preform. That method is described, for example, in the article by Le Sergent et al. entitled "Preform technologies for optical fibers" published in Electrical Communication, Vol. 62, No. 3/4, 1988, page 238.
Until now, the silica powders in use have been of two types.
The first type is a natural quartz powder of satisfactory grain size, but that may present random inclusions that are liable to make the subsequent preform fragile.
The second type is a synthetic powder or soot having a high degree of purity but in which the grains are so small (grain diameter less than 100 nm), that use thereof makes the POD method too lengthy and too expensive. The term "synthetic silica soot" is used below to designate this second type of powder. Such a soot may be obtained, for example, by an MCVD method; it may also be pyrogenic, like the soot sold under the name Aerosil OX-50 and 200 by Degussa.
European patent application EP-A-0 271 281 also describes a sol-gel method for making a silica powder for an optical fiber preform. That method consists in dispersing synthetic soot having a specific area of 200 square meters per gram (m.sup.2 /g) to a concentration of 25% by weight. The resulting gel is fractioned by mechanical means and the resulting grains are dried. It is observed that the grains are extremely friable, thereby making any subsequent screening operation difficult.